Phase transfer catalysts sulfur ylide reactions

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Recent work has indicated that solvents may even behave as phase transfer catalysts under certain conditions [50]. Generation of sulfur ylides by reaction of a sulfonium salt with a solid base was found to proceed via deprotonation of the solvent (Figure 5.21). It is believed that other weakly acidic solvents, such as rer -butanol and dimethylsulfoxide, behave in a similar manner, although it is not known whether these will function if they are present in just catalytic amounts. 

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